Way of seeing data as image anywhere in a Cycles nodes setup?
Personally, I use a temporary Emission shader and connect it to whatever I want to look at. I also have a small viewport render open almost permanently.
The emission shader renders really quickly taking only a couple samples to converge, so it's nearly instantaneous, especially if you go into local view (Numpad /).

I've recently released an add-on that uses this method, but sets it all up for you so you simply have to Ctrl+Shift+Left Click a node to view it's result (just like the compositor).



[Update: The Node Wrangler add-on mentioned is included in Blender now, simply enable it in your user preferences]